Since 1960, humans have tried 35 times to send missions to Mars. Depending on how you count, as many as 21 have failed. Spacecraft blew up on launch, never left Earth's orbit, crashed into the Martian surface, missed going into orbit and zoomed off into space, inexplicably shut down or simply disappeared.

"Mars is hard, and Mars is unpredictable," said Jim Graf of NASA's Jet Propulsion Laboratory in Pasadena, Calif. "Mars doesn't treat you very well."

Despite this record, Mars today has three satellites orbiting overhead, two rovers crawling around its surface -- and it appears destined to have an operating, robotic "human presence" in the neighborhood indefinitely.

The plan is to solve the mystery of how a world apparently once wet and warm turned into the chill, windblown wilderness that is Mars today. "We want to know whether it was habitable, whether life got started and, if it did, how it evolved," said geologist Raymond E. Arvidson of Washington University in St. Louis, who reviews the Mars program for NASA.

This Friday afternoon, barring a catastrophe like those that have doomed four missions in the past eight years, NASA's Mars Reconnaissance Orbiter will begin the rocket burn that will put it in orbit as the planet's fourth working satellite.

This is not a slam dunk. For a nail-biting half-hour during and after this process, the spacecraft will be out of contact behind Mars, while project manager Graf and the rest of the mission team wait to find out whether the planet has captured the new satellite or shot it back into space, never to return.

"We're on the money right now," Graf said at a recent news conference as the orbiter began its final approach. "But we are getting into the dangerous portion of the mission. A lot can go wrong, and if we don't succeed, we will fly right by the planet."

But win or lose, the human presence will endure. Functioning spacecraft have been on station at Mars since 1997, with new missions planned for launch in 2007, 2009 and beyond. And if humans ever actually land there to fulfill President Bush's "Vision for Space Exploration," they will have benefited from decades of research detailing such aspects as radiation hazards and the likeliest places to mine water ice.

"The missions we're doing for the next 10 years are pathfinders," said Arvidson, who chairs NASA's Mars Exploration Program Analysis Group. "We're doing the robotic precursor work -- picking out the best sites where humans on the surface can do the detailed studies."

At the dawn of the space age 50 years ago, many people still had the idea that Mars might be a habitable planet, not as threatening as the Mars of H.G. Wells's "The War of the Worlds" but a place that might harbor advanced life forms. "We were still making maps with canals on them," Arvidson recalled.

That archetype was dashed in 1965 when the flyby of the Mariner 4 spacecraft showed a barren, crater-pocked surface like Earth's moon. But over time, patient exploration revealed a much more complex landscape -- one perhaps sculpted in the past by flowing water.

Mars's three currently operating satellites -- NASA's Mars Global Surveyor, which arrived in 1997, NASA's Mars Odyssey and the European Space Agency's Mars Express -- developed this theory, and in 2004 the Mars rover Opportunity confirmed it by identifying sedimentary rock laid down in what had been a set of ancient lake beds.

"We have used the 'follow the water' strategy," said the Jet Propulsion Lab's Fuk K. Li, who oversees NASA's Mars exploration program. "In trying to understand whether life ever arose on Mars, water is key."

The Reconnaissance Orbiter will continue the theme. The spacecraft has a telescopic camera sensitive enough to spot boulders in ancient flood channels, a spectrometer to identify minerals formed by water processes, a "climate sounder" to measure atmospheric water vapor, and a radar capable of probing beneath Mars's surface in search of ice, or, possibly, liquid water.

"This is going to be a real intellectual leap forward," said Michael Meyer, NASA's lead scientist for the Mars program. "You can start looking at [geological] processes -- hot springs, gullies, volcanoes -- and we can spot many more minerals."

But first, the spacecraft has to get there safely. Launched from Cape Canaveral Air Force Station on Aug. 12, 2005, the $720 million Reconnaissance Orbiter has had up to now a flawless trip, needing only two course corrections to get into position for orbit insertion near Mars's south pole.

About 4:25 p.m. Eastern time Friday, the spacecraft's main thrusters will begin a pre-programmed 27-minute burn to reduce its speed by 2,200 mph, allowing Mars to capture it into orbit.

About 20 minutes later, the spacecraft will disappear behind Mars for a half-hour. During that time the burn will end, but engineers will not know the fate of the mission until they regain radio contact.

At that point, if all goes well, the Reconnaissance Orbiter will be in a highly elliptical, near-polar orbit, with its closest approach 249 miles above the south pole, and its farthest point at 27,340 miles from the north pole.

After systems and instrument checkouts, engineers will begin "aerobraking" by the end of March, dipping the spacecraft into the Martian atmosphere on every pass to slow it down and bring it into a much tighter ellipse.

This will take about six months and "must be done in a controlled fashion," Graf said, because upward "spikes" in the Martian atmosphere could increase friction on the satellite, causing it to overheat. Mistaking English measurements for metric measurements during aerobraking caused NASA's Mars Climate Orbiter to break up in the Martian atmosphere in 1999.

Current plans call for the Reconnaissance Orbiter to settle into a working "science orbit" between 199 and 158 miles above the Martian surface and spend two years surveying, mapping, hunting for water and investigating landing sites for future Mars missions.

At the end of that time, engineers will shift the spacecraft into a higher orbit from 217 to 255 miles above the surface, where it will serve primarily as a communications relay. The orbiter has more data capacity than the rest of the Mars satellite fleet combined, Meyer said, holding "about as much information as there is in a video store."

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Disclaimer: Vera Pinto Gomes is a space policy analyst currently working at the European Satellite Navigation Programs (Galileo & EGNOS) at the European Commission. She is also a member of the History Committee of the International Academy of Astronautics. Views and opinions expressed in this blog are those of the author alone. Sharing posts does not mean endorsement
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